Liquid crystal displays, in particular liquid crystal displays adopting thin film transistors (TFTs), have been widely used in various applications from mobile phones to large-sized televisions.
One application is a personal display device, which has a display screen generally intended to be seen by a user of the personal display device but not by other persons who may view the device from the side.
Preferably, the personal display device is constructed such that the display screen of the device can be viewed by a large number of persons or the display screen can be viewed exclusively by only one individual, as occasion demands.
FIG. 11 is a view illustrating a conventional liquid crystal display 100 having a secret mode 130.
There has been proposed a display 100 having the secret mode 130 shown in FIGS. 11A-11C. (for example, Japanese Unexamined Publication No. 5-72529).
Referring to FIG. 11A, a backlight 120 for emitting light to a liquid crystal panel 100 from the rear side has high directionality.
Between the common liquid crystal panel 100 and the directional backlight 120 is disposed another liquid crystal panel 110 for switching between a scattered state 140 and an unscattered state 130, for example, a polymer-type liquid crystal panel (a scattering-unscattering switching panel).
Referring to FIG. 11B, when the scattering-unscattering switching panel 110 is in an unscattered state 130, light from the backlight 120 is emitted to only the front, and therefore, it is not possible to see an inclined display.
On the other hand, referring to FIG. 11C, when the scattering-unscattering switching panel 110 is in a scattered state 140, light from the backlight 120 is emitted in an inclined direction, and therefore, it is possible to see an inclined display. Consequently, a large number of persons can view the display.
In this case, it is necessary to manufacture a special liquid crystal panel 110 in addition to the common liquid crystal panel 100, and therefore, the manufacturing costs are increased.
In order to solve this problem, a method using a vertical alignment type liquid crystal display has been proposed.
Hereinafter, the fundamental principle thereof will be described in detail with reference to FIGS. 12A to 15.
FIGS. 12A and 12B are views illustrating the shape of a liquid crystal molecule 200 when viewing the vertical alignment type liquid crystal display from the front (z-direction) when no voltage is applied (FIG. 12A) and when a voltage is applied (FIG. 12B).
As shown in FIG. 12A, the liquid crystal molecule 200 is oriented vertically (along the z-direction) when no voltage is applied. When the voltage is applied as shown in FIG. 12B, the liquid crystal molecule 200 is inclined upward.
A polarizer and an analyzer are directed to their absorption axes 210, 220, respectively.
FIG. 12A illustrates a case in which the vertically oriented liquid crystal panel is viewed from the front when the voltage is not applied. Double refraction of the liquid crystal molecule 200 does not occur, and light leakage may be avoided.
On the other hand, FIG. 12B illustrates a case in which the vertically oriented liquid crystal panel is viewed from the front when the voltage is applied. The optical axis of the liquid crystal molecule 200 may be parallel with the absorption axis of the polarizer 210. Double refraction of the liquid crystal molecule may not occur, and light leakage may be avoided.
FIGS. 13A and 13B are views illustrating the shape of a liquid crystal molecule 200 when viewing the vertical alignment type liquid crystal display from the side (at an angle to the front) of the liquid crystal display.
When the voltage is not applied, as shown in FIG. 13A, the axis of the liquid crystal molecule 200 is parallel with the absorption axis of the analyzer 220, and therefore, light leakage may be avoided.
On the other hand, when the voltage is applied, as shown in FIG. 13B, the axis of the liquid crystal molecule 200 may be offset from the axis of the polarizer 210 or the axis of the analyzer 220. Consequently, double refraction of the liquid crystal molecule 200 may occur, and light may leak.
When light leakage occurs, the display contrast is lowered to the extreme in the side direction. As a result, it is not possible to recognize what is written even when the display is seen from the side. Consequently, it is possible to control the confidentiality of the display by using this light leakage phenomenon.
FIG. 14 is a view illustrating a specific construction for controlling the confidentiality of the display.
Referring to FIG. 14, a single pixel 300 includes sub-pixels of red 310, green 320 and blue 330 (RGB) and a sub-pixel of white 340 (W).
FIG. 15 is a view illustrating the arrangement of liquid crystal molecules 200 within the respective sub-pixels shown in FIG. 14. As shown in FIG. 15, the orientation of the liquid crystal molecules 200 in the white sub-pixel 340 is quite different from the orientation of the liquid crystal molecules in the RGB sub-pixels 310, 320, 330. Specifically, the liquid crystal molecules 200 are oriented upward and downward in the white sub-pixel 340.
Consequently, when the voltage is not applied to the white sub-pixel 340, the white sub-pixel 340 does not contribute to the display, and therefore, a normal display can be realized.
On the other hand, when the voltage is applied to the white sub-pixel 340, the white display is revealed at the front in the horizontal direction. As a result, the contrast of the display is lowered in a horizontal viewing angle orientation, and therefore, it is difficult for other people to view the display.
Hereinafter, the display control in the conventional vertical alignment type liquid crystal display will be described.
FIG. 16 is a plan view illustrating an enlarged pixel 405 of the conventional vertical alignment type liquid crystal display. A common line 430, a signal line 435, and a gate line 440 are also shown. And, FIG. 17 is a sectional view illustrating the enlarged pixel of the conventional vertical alignment type liquid crystal display. In addition, FIGS. 18A and 18B are views illustrating the operation of liquid crystal molecules 200 due to the application of voltage in the conventional vertical alignment type liquid crystal display.
A generally “<”-shaped common electrode 400 for prescribing the liquid crystal inclination direction is formed on a transparent electrode 410 at a color filter 420 side (see FIGS. 16 and 17).
When the voltage is not applied, the liquid crystal molecules 200 are oriented in an upward direction, as shown in FIG. 18A.
When the voltage is applied, on the other hand, the liquid crystal molecules 200 are oriented in the direction determined by the inclined electric field of the common electrode 400, i.e., the direction perpendicular to the spreading direction of the common electrode 400, as shown in FIG. 18B. As a result, a liquid crystal display having a good viewing angle may be realized.
However, the conventional liquid crystal display has the following problems.
First, the conventional liquid crystal display is constructed such that the white sub-pixel is formed; consequently, a white resin must be formed, and the driving operation of the white sub-pixel is different from the conventional art.
Second, the contrast may be lowered in the horizontal orientation, but the contrast may not be lowered in the vertical orientation.